Structuring System Logical Requirements

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Modern Systems Analysisand DesignFourth
Edition

• Chapter 8
• Structuring System Logical Requirements

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Learning Objectives

• Use structured English as a tool for representing
  steps in logical processes in data flow diagrams.
• Use decision tables and decision trees to
  represent logical choice in conditional
  statements.
• Select among structured English, decision tables,
  and decision trees.

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Logic Modeling

• Data flow diagrams do not show the logic inside
  the processes.
• Logic modeling involves representing internal
  structure and functionality of processes depicted
  on a DFD.
• Logic modeling can also be used to show when
  processes on a DFD occur.

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Logic Modeling Deliverables and Outcomes

• Structured English
• Decision Tables
• Decision Trees
• State-transition diagrams
• Sequence diagrams
• Activity diagrams

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Modeling Logic with Structured English

• Modified form of English used to specify the
  logic of information processes
• Uses a subset of English
• Action verbs
• Noun phrases
• No adjectives or adverbs
• No specific standards

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Structured English is used here to describe input
and output.
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Structured English is used here to describe
arithmetic operations.
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Structured English is used here to describe
repetition.
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Structured English is used here to describe
decisions.
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Structured English is used here to describe
invoking other processes.
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Modeling Logic with Decision Tables

• A matrix representation of the logic of a
  decision
• Specifies the possible conditions and the
  resulting actions
• Best used for complicated decision logic

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3 Parts of a Decision Table

• Condition stubs
• Lists condition relevant to decision
• Action stubs
• Actions that result from a given set of
  conditions
• Rules
• Specify which actions are to be followed for a
  given set of conditions
• Indifferent Condition
• Condition whose value does not affect which
  action is taken for two or more rules

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Procedure for Creating Decision Tables

• Name the condition and values each condition can
  assume
• Name all possible actions that can occur
• List all rules
• Define the actions for each rule
• Simplify the table

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Decision Table
Note for salaried employees the action stub
chosen will always be the sametherefore hours
worked is an indifferent condition
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Reduced Decision Table
Because of indifferent condition, the complete
decision table can be reduced to one with fewer
rules
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Modeling Logic with Decision Trees

• A graphical representation of a decision
  situation
• Decision situation points are connected together
  by arcs and terminate in ovals
• Main components
• Decision points represented by nodes
• Actions represented by ovals
• Particular choices from a decision point
  represented by arcs

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Modeling Logic with Decision Trees (cont.)

• Read from left to right
• Each node corresponds to a numbered choice on a
  legend
• All possible actions are listed on the far right

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Decision tree representation of salary decision
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Alternative decision tree representation of
salary decision
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Deciding Among Structured English, Decision
Tables, and Decision Trees
Criteria Structured English Decision Tables Decision Trees
Determining Conditions and Actions Second Best Third Best Best
Transforming Conditions and Actions into Sequence Best Third Best Best
Checking Consistency and Completeness Third Best Best Best
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Deciding Between Decision Tables and Decision
Trees
Criteria Decision Tables Decision Trees
Portraying complex logic Best Worst
Portraying simple rules Worst Best
Making decisions Worst Best
More compact Best Worst
Easier to manipulate Best Worst
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Summary

• In this chapter you learned how to
• Use structured English as a tool for representing
  steps in logical processes in data flow diagrams.
• Use decision tables and decision trees to
  represent logical choice in conditional
  statements.
• Select among structured English, decision tables,
  and decision trees.